The present invention relates to an improved process for imparting stability against decomposition to particulate vinylidene chloride copolymer resins and to the particulate vinylidene chloride resins by the improved process. As used herein, the term vinylidene chloride includes polyvinylidene chloride as well as copolymers and interpolymers having polymerized therein at least a major portion of vinylidene chloride.
Vinylidene chloride polymers and methods of preparation thereof are widely reported in patents and other literature. Perhaps as widely reported are the tendencies of vinylidene chloride polymers to discolor and to cross-link and gel when subjected to elevated temperatures, particularly when in the presence of even trace amounts of certain metal salts, particularly iron salts. The metal salt impurities may be present as a result of contact of the polymer with metal polymerization or processing equipment, or may be acquired from water used as the polymerization medium or are present as impurities in various modifiers added to the polymer. Such impurities are not easily removed from the polymer by usual washing methods.
An effective stabilizer for haloethylene polymers in general and vinylidene chloride polymers in particular must satisfy the following requirements, especially when the polymers are subjected to the presence of metal salts: (1) react with evolved hydrochloric acid to prevent attack on iron or other metal with which the polymer comes in contact; (2) react with an iron (or other metal) and dissolve or precipitate such metal as an insoluble compound; and (3) react as a chain terminating agent or scavenger for polymer free radicals.
A particular stabilizer composition may have a number of ingredients. Some ingredients, when used alone and in excess, will not only fail to impart stability to the polymer but will actually hasten, or contribute to, degradation thereof. Accordingly, a careful balancing of stabilizer composition ingredients and amounts thereof is required. Known stabilizers, some of which may have more than one function, include organic polyhydric phenols, organic phosphites, metal salts of fatty carboxylic acids, polyphosphates, alkyltin polysulfide thioesters and other organo-tin compounds, lead compounds, epoxy compounds, thiodipropionic acid esters, mono-nuclear aromatic quinone compounds and the like.
It is known that many stabilizers exhibit more than one function. Epoxides, for example, are believed to act both as hydrochloric acid acceptors and as chelating agents. They may also have some antioxidant activity. Other acid acceptors commonly used include magnesium oxide, tetrasodium pyrophosphate, metal salts of fatty carboxylic acids and organo-tin compounds.
It is known that alkali metal polyphosphate salts, such as tetrasodium pyrophosphate in a very finely ground form have been dry blended with film-forming particulate vinylidene chloride polymer resins. This has been less than satisfactory for a number of reasons. First, the finely ground alkali metal polyphosphate salts have a light powdery consistency and are hydroscopic, a combination which favors formation of agglomerates. Second, any agglomerates which do form are difficult to filter out because the powdered alkali metal polyphosphate salts tend to clog or blind filter screens. Third, those agglomerates which are not filtered out tend to show up as white specks in extruded tape or film formed from the vinylidene chloride copolymer resin. The white specks are unsatisfactory not only from an appearance standpoint but from a film performance standpoint as some agglomerates or white specks are large enough to cause formation of holes in the film. Fourth, notwithstanding the potential for agglomerate formation, there is also a problem of ensuring generally uniform distribution of the salts upon the resin particles. Such a problem may result in some polymer resin having no stabilizer salts in contact therewith.
It is also known that after dry blending such alkali metal polyphosphate salts with the film-forming particulate vinylidene chloride polymer resins, plasticizers such as epoxidized soybean oil are added by conventional techniques to aid in processing of the resins. While such plasticizers admittedly aid in resin processing, they generally do not aid in dispersion or distribution of previously added particulate or powdery stabilizers such as the alkali metal polyphosphate salts.